1. Field of the Invention
This invention relates to improvements in an elastic surface wave filter which is composed of a substrate for the propagation thereon of elastic surface waves and transmitting and receiving transducers, each having at least an electrode deposited on the major surface of the substrate and in which the transmitting and receiving transducers are disposed on the major surface of the substrate a predetermined distance a part from each other, the electrode of one or both of the transmitting and receiving transducers is formed with first and second comb-shaped electrodes and the first and second comb-shaped electrodes are disposed with each electrode element of the latter lying between adjacent ones of the electrode elements of the former.
2. Description of the Prior Art
There are heretofore been proposed an elastic surface wave filter of this kind in which electrode elements of first and second comb-shaped electrodes of a transducer are not weighted in terms of length. Such an elastic surface wave filter is called an elastic surface wave filter provided with a transducer having non-weighted electrode. In the case of the non-weighted electrode, the region of overlapping i.e. interdigating region of electrode elements of first and second comb-shaped electrodes (which region will hereinafter be referred to as the overlapping region) is rectangular with sides respectively parallel and perpendicular to a line connecting the transmitting and receiving transducers (the direction of which line will hereinafter be referred to as the first direction). In the conventional elastic surface wave filter provided with the transducer having such a non-weighted electrode, the relative minimum attenuation (an insertion loss) in the pass band is relatively small but that in the rejection band is also relatively small.
Further, there has also been proposed an elastic surface wave filter of this kind in which the electrode elements of the first and second comb-shaped electrodes of the transducer are weighted in terms of length. Such an elastic surface wave filter is called an elastic surface wave filter provided with a transducer having a weighted electrode. With such a conventional weighted electrode, the shape of the region of overlapping of the electrode elements of the first and second comb-shaped electrodes is such that its extent along a line parallel to the first direction varies with displacement of the line in a second direction, i.e. a direction normal to the first direction, so as to pass through different points along the length of an electrode element. In the conventional elastic surface wave filter provided with the transducer having such weighted electrode, the relative minimum attenuation in the rejection band is relatively large and that in the pass band is also relative large.
Accordingly, in such conventional elastic surface wave filters, whether the electrode is weighted or not, it is difficult to make insertion loss in the pass band relatively small and attenuation in the rejection band relatively large, so that these conventional elastic surface wave filters cannot be said to have excellent band-pass characteristic.